However it turned out that the character, van Lutyens, was supposed to be Dutch; he even finishes his first scene with a Verdomme! (recorded in the script as "mutters his reply in Dutch"). The actor, John Abineri, was apparently fluent in German, but that doesn't explain the accent in this case.
I suppose it's simply that to a British audience in 1968 (and indeed to a British actor) a South African accent sounded more realistically "Dutch" than a Dutch accent would have done. This was, after all, years before the UK joined the EEC. (I never saw "Van der Valk", which started in 1972; did its Dutch characters have accents? Or was it all played as if everyone spoke perfect English? Which of course is not totally unrealistic for Amsterdam.)